Method of obtaining polycrystalline silicon and workpiece useful therein

ABSTRACT

Polycrystalline silicon is obtained by providing a silicon wafer having disposed over at least one face thereof a base coating of oxide, nitride or oxynitride composition, forming a substantially pinhole-free and scratch-free layer of carbon on said base coating over at least the face, forming on the face of the carbon layer a layer of polycrystalline silicon, and removing the silicon layer from the protective coating. Any of the carbon layer adhering to the silicon layer is easily removable to provide the silicon layer separate from the substrate. The wafer/coating unit is reusable in the procedure. The wafer/coating/carbon layer unit comprises a workpiece useful in the practice of the invention.

This is a division of application Ser. No. 037,864, filed May 10, 1979.

BACKGROUND OF THE INVENTION

The present invention relates to a method of obtaining polycrystallinesilicon, and more particularly to a method of separating polycrystallinesilicon from the substrate on which it is grown, and a workpiece usefulin the practice of the method.

Sheets of polycrystalline silicon are used for many applications, one ofthe applications being in solar cells or photovoltaics. A major factorlimiting the use of such sheets in solar cells is the high cost ofobtaining a sheet separated from the substrate upon which it is formed.For example, formation of a sheet directly on an expensive highlypolished silicon wafer substrate makes it impossible to separate thesheet from the substrate. At the very least, the substrate must becleaned and repolished after each use.

In an attempt to ameloriate this problem, the wafer has been providedwith an oxide, nitride or oxynitride coating and then the sheet formeddirectly on top of the coating. This has not proven satisfactory as thesheet adheres tightly to the coating and it is almost impossible toremove the sheet from the coating without damaging both the sheet andthe coating.

It is known that germanium grown on a thick layer of carbon over asubstrate is easily separable from the quartz substrate. In an attemptto ameloriate the problem described above by use of this approach, ahighly polished silicon wafer has been coated with carbon and then asheet of polycrystalline silicon formed directly on top of the carbonlayer. While this approach enabled easy separation of the sheet from thecarbon layer when the carbon layer was essentially free from pinholes,an essentially pinhole-free carbon layer could be obtained only when thecarbon layer was so thick that it was difficult to obtain the necessaryflatness in the upper surface thereof and the process was economicallyunattractive due to the increased power requirements for heating thewafer/carbon unit during the growth step. Furthermore, it was difficultto remove the carbon adhering to the wafer after sheet separation,possibly due to the formation of silicon carbide compounds. In brief,this approach was not suitable for mass production techniques.

Accordingly, it is an object of the present invention to provide amethod of obtaining a sheet of polycrystalline silicon which enableseasy separation of the sheet from the substrate wafer.

Another object is to provide such method in which the substrate, or atleast a major portion thereof, is reusable.

A further object is to provide such a method which is economical andadapted to mass production techniques.

A final object is to provide a workpiece which is useful in the practiceof such a method and enables easy separation of the sheet from thesubstrate wafer.

SUMMARY OF THE INVENTION

It has now been found that the above and related objects of the presentinvention are obtained by providing a substrate comprising a waferhaving a base coating of oxide, nitride or oxynitride on top thereof,and forming a layer of carbon on top of the coating and then a layer ofpolycrystalline silicon on top of the carbon layer. The silicon layer isthen easily separable from the wafer and base coating--for example, bywedging one or more thin objects substantially intermediate the siliconlayer and the base coating.

While it has been learned that neither the base coating nor the carbonlayer by itself enables easy removal of the silicon layer from thewafer, and that the combination of the two does enable such easyremoval, it is not fully understood why the combination of the two isoperable and each individually is not.

More particularly, the method of obtaining polycrystalline siliconcomprises the steps of providing a substrate body having a substantiallyplanar face and a sidewall. A base coating of a composition selectedfrom the group consisting of oxide, nitride and oxynitride compositionsis disposed over at least the entire substrate body face, and preferablyalso the sidewall thereof. A substantially pinhole-free and scratch-freelayer of carbon is formed on the base coating over at least the entireface thereof, and preferably also the sidewall thereof. A layer ofpolycrystalline silicon is then formed on the face of the carbon layer.Finally, the silicon layer is removed from the protective coating.

Any of the carbon layer adhering to the base coating face is removed andthe procedure repeated, starting with reconstitution of the carbonlayer. Any of the carbon layer adhering to the silicon layer is removedin order to provide a silicon layer or sheet free from its substrate.

The method described above may be repeated a plurality of times untilthe base coating deteriorates, at which point the base coating isreconstituted and the procedure repeated.

In a preferred embodiment, in order to separate the silicon layer andthe base coating, a thin object is wedged between the silicon layer andthe base coating, preferably immediately below the portion of thesilicon layer abutting the carbon layer face.

Preferably the carbon layer is formed by exposing the base coating faceand sidewall to the fumes of ignited xylene, or, alternatively, byheating and exposing to xylene the base coating face and sidewall.

Preferably the substrate body is formed of silicon, the substrate bodyface is highly polished, the base coating is substantially pinhole-freeand scratch-free, and the carbon layer is of uniform thickness.

The workpiece of the present invention comprises a substrate body havinga substantially planar upper face and a sidewall, a base coating of theaforementioned composition disposed over at least the entire body face(and preferably also the sidewall thereof), and a substantiallypinhole-free and scratch-free layer of carbon disposed over at least theentire exposed base coating face (and preferably also the sidewallthereof). In a later stage of the method of the present invention theworkpiece further includes a layer of polycrystalline silicon disposedover at least the exposed face of the carbon layer.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a front elevation view of a silicon wafer having a basecoating thereon, according to the principles of the present invention;

FIG. 2 is a front elevation view of the wafer/coating unit with a layerof carbon disposed thereon;

FIG. 3 is a front elevation view of the wafer/coating/carbon unit with alayer of polycrystalline silicon disposed thereon;

FIG. 4 is a front elevation view similar to FIG. 3, but showing thesilicon layer being removed from the wafer/coating unit; and

FIG. 5 is a front elevation view of the silicon layer removed from itssubstrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawing, and in particular to FIG. 1 thereof,therein illustrated is a substrate body in the form of a silicon wafer,generally designated 10. Preferably the wafer 10 has the substantiallyplanar upper face 12 thereof, and typically the sidewall 14, highlypolished to provide a flat mirror-like finish. The backside 16 thereofmay also be highly polished. The wafer 10 will generally be of uniformthickness (about 250-1000 microns) and may be formed with any suitablediameter (for example, about 15 centimeters).

A base coating, generally designated 20, is disposed at least over theentire wafer face 12 and preferably also over the sidewall 14 thereof.In order to complete sealing of the wafer 10, a useful procedure wherethe wafer 10 contains dopants, the base coating may also be disposedover the entire wafer backside 16, thereby to encapsulate the wafer 10.The coating 20 is desirably substantially pinhole-free and scratch-freeas well as of uniform thickness (generally about 2000-4000 Angstroms).The composition of the coating may be oxide, nitride, or oxynitride. Thetechniques for depositing such a coating on a wafer are well known inthe art and need not be described in detail herein. See, for example,"Silicon Nitride Chemical Vapor Depositions in a Hot Wall DiffusionSystem," J. Electrochem. Soc., Volume 125, No. 9, pages 1557-1559(September 1978); "Preparation and Some Properties of ChemicallyVapor-Deposited Si-rich SiO₂ and Si₃ N₄ Films", J. Electrochem. Soc.,Volume 125, No. 5, pages 819-822 (May 1978); "Composition, ChemicalBonding, and Contamination of Low Temperature SiO_(x) N_(y) InsulatingFilms," J. Electrochem. Soc., Volume 125, No. 3, pages 424-430 (March1978); "Improved Theoretical Predictions For the Steam Oxidation ofSilicon at any Elevation," J. Electrochem. Soc., Volume 125, No. 9,pages 1514-1517 (September 1978); and "Chemical Vapor Deposition ofSilicon Nitride," J. Electrochem. Soc., Volume 125, No. 9, pages1525-1529 (September 1978). The coating 20 not only provides a top face22 and sidewall 24 from which the carbon layer later applied theretowill be easily removable, but it also aids in sealing the wafer 10 toprotect the polycrystalline silicon sheet later grown thereon from thedeleterious effects of pinholes in the carbon layer and outgassing ofthe wafer 10 under growth conditions.

Referring now in particular to FIG. 2, a substantially pinhole-free andscratch-free layer of carbon, generally designated 30, is then formedover at least the entire upper face 22 of the coating 20, and preferablyalso the sidewall 24 thereof. The carbon layer 30 is preferably ofuniform thickness, thereby to provide a flat upper face 32 on which thepolycrystalline silicon sheet can later be grown as well as a sidewall34 on which some of the polycrystalline silicon may also form. Thecarbon layer 30 is extremely thin, preferably 12-380 microns inthickness. If the carbon layer is too thick, too much power is requiredto bring it up to the temperature required for chemical vapor depositionof the polycrystalline silicon sheet and it is difficult to insureuniform flatness of the upper surface 32 on which the sheet is grown. Ifthe carbon layer 30 is too thin, it tends not to be substantiallypinhole-free, rendering it difficult to separate the polycrystallinesilicon sheet later grown thereon from the coating 20 as describedhereinbelow. The carbon layer 30 may also be applied over the basecoating on the wafer backside 16, but this is neither necessary noruseful as ordinarily the polycrystalline silicon will not form on thewafer backside.

A variety of different techniques useful in forming the carbon layer 30will be readily apparent to those skilled in the chemical vapordeposition art. For example, analytical pyrolizing reagent-grade xylenemay be ignited under carbonizing conditions. The wafer/protectivecoating unit may then be held on the wafer backside 16 (for example, bya vacuum chuck) and the front face 22 of the wafer/coating unit passedevenly over the flame to provide a thin uniform carbon layer 30. Ifnecessary, the wafer should be tilted slightly from one side to anotherto insure that carbon layer 30 is also formed on the coating sidewall24. Alternatively, the wafer/coating unit may be either rested with itsbackside 16 lying on a susceptor or suspended with its backside 16 heldby a vacuum chuck, and the unit then exposed to a stream of an inertcarrier gas which has been bubbled through xylene. In this instance thewafer/coating unit should be heated by conventional means as necessaryto maintain the unit at the proper temperature for pyrolitic carbonformation. The carbon layer 30 may also be formed by other techniquessuch as dipping the appropriate surfaces into carbon powder (graphite)or spraying the appropriate surfaces with an emulsified carbon bath orapplying a graphite solution to the appropriate surfaces with a spinner,provided in all instances that the carbon layer 30 thus formed issubstantially pinhole-free and scratch-free, and not deleteriouslycontaminated (e.g., by emulsifiers, solvents and the like), andsufficiently adherent to the coating 20 so that it is not entirely blownaway by the gases passing thereby during the later sheet formation step.The wafer/coating/carbon layer unit constitutes the basic workpieceuseful in the practice of the method of the present invention.

Referring now to FIG. 3 in particular, a layer or sheet 40 ofpolycrystalline silicon is then formed on the upper face 32 of thecarbon layer 30, for example, by conventional techniques well known tothose skilled in the epitaxy and chemical vapor deposition arts. See,for example, "The Fundamentals of Chemical Vapour Deposition," Journalof Material Sciences, 12 (Chapman & Hall Ltd. 1977), pp. 1285-1306. Thesheet 40 will typically extend downwardly over the carbon layer sidewall34, but in a thinner layer. The sheet is generally 250-750 microns thickatop the carbon layer face 32. Thinner sheets have a tendency to warp orbreak as the sheet is being removed from the substrate as describedhereinbelow, while thicker sheets are not economical and are difficultto form with the desired degree of uniform thickness.

Referring now to FIG. 4 in particular, the polycrystalline silicon sheet40 is then removed from the wafer/coating unit. As shown, a plurality ofsubstantially uniformly spaced thin objects 50 are wedged intermediatethe sheet backside 52 and the coating upper surface 22. The objects 50may be razor blades or the like having a thickness on the order of about12 microns. The objects 50 are preferably inserted immediately below thesilicon sheet backside 52 abutting the carbon layer face 32. The task ofknowing where to position the objects 50 is simplified by the fact thatthe polycrystalline silicon sheet 40 is gray whereas the carbon layer 30is black. The actual separation task is simplified by the fact that thepolycrystalline silicon layer 40 tends to be rather thin along thecarbon layer sidewall 34.

Other techniques presently contemplated for use in separating the sheet40 from the protective coating 20 include the use of ultrasonics andthermal shock, for example, by rapid chilling of the wafer/coating unitwith liquid nitrogen or rapid heating of the wafer/coating unit. Theefficacy of these techniques, of course, depends upon the low level ofadhesion of the sheet 40 to the coating 20 due to the presence of theintermediate carbon layer 30.

Regardless of the specific technique utilized to separate the sheet 40from the coating 20, there is likely to be a certain amount 30a of thenow destroyed carbon layer 30 adhering to the sheet backside 52 and acertain amount 30b adhering to the coating upper face 22 and sidewall24. The carbon 30a adhering to the sheet backside 52 may be removed bysandblasting, an acid dip (e.g., using acetic, nitric and hydrofluoricacids), grinding, ultrasonics, a combination thereof, or by othertechniques well recognized in the art for removing carbon from a siliconsheet. The silicon sheet is then available for use, separate from itsformer substrate, as desired.

The carbon 30b adhering to the coating upper face 22 is exposed, but thecarbon 30b adhering to the coating sidewall 24 is covered by a thinlayer of polycrystalline silicon 40. The thin layer of silicon isremoved first, for example, by careful scraping to insure that thecoating sidewall 24 is not damaged (although it is immaterial whether ornot the intermediate carbon layer sidewall 34 is damaged). Then thecarbon 30b is easily removed by simple soft nylon brushing and/ordeionized water washing, care being taken to make sure that the carbonremoval procedure does not injure the underlying coating 20. Thewafer/coating unit is then available for reuse, and the procedure may berepeated starting with formation of the carbon layer 30 on the coating20, as described hereinabove, to reconstitute the basic workpiece of thepresent invention. It has been found that the procedure may be repeatedmany times using the same wafer/coating unit so that the cost of theunit is amortizable over the many silicon sheets obtained by usethereof, this rendering the process economical. If the coating 20 of thewafer/coating unit becomes scratched or otherwise damaged, it is asimple and relatively inexpensive procedure to remove the damagedcoating 20 from the wafer 10 and then to apply a fresh coating 20 to thewafer 10, thereby to reconstitute the coating 20 and enable theprocedure to restart.

To summarize, the present invention provides an economical process forobtaining a sheet of polycrystalline silicon which enables easyseparation of the sheet from the substrate wafer, the substrate waferbeing undamaged and reusable in the process, thereby providing aneconomical mass production technique for obtaining polycrystallinesilicon sheet separated from the substrate on which it is grown. Thepresence of both a base coating and a carbon layer intermediate thesubstrate wafer and the grown polycrystalline silicon facilitates theseparation process.

Now that the preferred embodiments have been shown and described indetail, various modifications and improvements thereon will becomereadily apparent to those skilled in the art. For example, while thepreferred embodiments have been described in terms of a siliconsubstrate body because only silicon is currently known to be suitablefor use as the substrate body in a chemical vapor deposition system forthe growth of polycrystalline silicon, the principles of the presentinvention are equally applicable to substrate bodies formed of materialsother than silicon which also meet the requirements for a substrate bodyuseful in a chemical vapor deposition system for the growth ofpolycrystalline silicon. Accordingly, the spirit and scope of thepresent invention is to be limited only by the appended claims, and notby the foregoing disclosure.

We claim:
 1. A workpiece useful in the chemical vapor deposition ofpolycrystalline silicon comprising:(A) a solid substrate body having asubstantially planar upper face and a sidewall; (B) a base coatingdisposed over at least said substrate body face, said base coating beingselected from the group consisting of oxide, nitride and oxynitridecompositions, and (C) a substantially pinhole-free and scratch-freelayer of carbon disposed over at least the face of said base coating andadapted to receive polycrystalline silicon thereon.
 2. The workpiece ofclaim 1 wherein said base coating is disposed at least over saidsubstrate body face and the sidewall thereof, and said carbon layer isdisposed at least over said base coating face and the sidewall thereof.3. The workpiece of claim 2 wherein said base coating encloses theentire surface of said substrate body.
 4. The workpiece of claim 2additionally including a layer of polycrystalline silicon disposed overat least said carbon layer face.
 5. The workpiece of claim 2 whereinsaid base coating is substantially pinhole-free and scratch-free.
 6. Theworkpiece of claim 2 wherein said substrate body face is highlypolished.
 7. The workpiece of claim 2 wherein said carbon layer on saidbase coating face is of uniform thickness.
 8. The workpiece of claim 2wherein said substrate body is formed of silicon.